1. Field of the Invention
The present invention relates to an ink jet print head that ejects ink droplets to print media, and in particular, to an ink jet print head suitable for high-speed and high-definition printing.
2. Description of the Related Art
With the increased operation speed and improved image quality of ink jet printing apparatuses, attempts have been made to reduce the size of droplets ejected from a print head and to increase ejection frequency.
The opening area of each ejection port in the print head needs to be reduced in order to reduce the size of ejected droplets. However, the reduced opening area of the ejection port increases the flow resistance of a liquid in a portion (ejection port portion) that communicates with the ejection port. This may prevent desired ejection performance and efficiency from being obtained. Thus, ink jet print heads are disclosed in Japanese Patent Laid-Open Nos. 2004-042651 and 2004-042652 as a method of reducing the flow resistance of the ejection port portion while maintaining the strength of an ejection port forming portion.
The ink jet print heads disclosed in Japanese Patent Laid-Open Nos. 2004-042651 and 2004-042652 each have a plurality of nozzles through which ink flows. As shown in FIG. 8, each of the nozzles has a supply channel 39 through which ink is supplied, a bubbling (bubble) chamber 38 in which the ink is heated to generate bubbles, and an ejection port portion 36 including an ejection port 37 that is a tip opening of a nozzle through which ink droplets are ejected. The ejection port portion 36 allows the ejection port 37 and the bubbling chamber 38 to communicate with each other and includes a first ejection port portion 36a and a second ejection port portion 36b that communicate with the ejection port 37. The first ejection port portion 36a and the second ejection port portion 36b constitute a cylindrical space centered around one central axis S0 passing through the center of an electrothermal conversion element 34 orthogonally to a major surface 32a of an element substrate 32. Thus, in the ejection port portion 36, an area R31 located inwardly in an ink supply direction (X direction) and an area R32 located outwardly in the same direction are symmetrical with respect to the central axis S0. Furthermore, if the second ejection port portion 36b is cut in a direction parallel to the major surface 32a, the resulting opening portion is located outside an opening resulting from cutting of the first ejection port portion 36a in the same direction and inside a cross section of the bubbling chamber 38 in the same direction. That is, the second ejection port 36b constitutes a space obtained by enlarging the first ejection port portion 36a in a plane direction.
In the thus configured ink jet print head 30, the strength of a peripheral portion of the ejection port 37 can be determined based on the thickness of the first ejection port portion 36a. Furthermore, the space enlarged by the second ejection port portion 36b reduces the flow resistance of the entire ejection port portion. This enables a possible pressure loss in the ejection port portion 36 to be reduced to allow bubbles to grow (expand) in an ejection direction. As a result, the ejection efficiency can be improved.
Furthermore, the amount of time from immediately after ejection until the start of refilling of ink in the bubbling chamber (hereinafter referred to as the refill time) needs to be reduced in order to increase the ejection frequency at which ink droplets are ejected from the ink jet print head. To reduce the refill time, the ink supply channel needs to be designed to have an increased channel width and an increased height to reduce the flow resistance to the ink in the ink supply channel.
However, the following has become clear. The enlarged ink supply channel results in the formation of a large space in the bubbling chamber, composed of a part of the ink supply channel. As a result, fine ink droplets (satellites) are generated which do not contribute to image formation during ejection of ink droplets.
FIG. 9A shows a stage in which, after a main droplet D1 is ejected through the ejection port 37 by means of a bubble B generated by heat from an electrothermal conversion element H, the bubble is eliminated. In FIG. 9A, M denotes a meniscus made up of ink forming the bubble. D2 denotes a trailing portion of the droplet coupled to the meniscus.
As the bubble becomes smaller over time as shown in FIG. 9B, the meniscus is partly ruptured and the bubble starts to communicate with the atmosphere. The trailing portion D2 is coupled to ink positioned outwardly in the channel in the bubbling chamber. Finally, the trailing portion D2 is severed from the ink in the bubbling chamber R to form an ink droplet completely separate from and independent of the ink in the channel.
At this time, if the ink channel is set to have a large volume in order to increase the ink refill speed, the trailing portion D2 becomes long, and the severance of the trailing portion D2 from the ink in the bubbling chamber R is likely to generate a fine droplet (satellite) D3 split from the trailing portion D2. The satellite is emitted though the ejection port to the exterior, where the satellite creates a mist. The ink mist adheres to a print medium, the print head, and unspecified areas in the ink jet printing apparatus main body, posing problems such as degraded print quality or malfunctioning of the printing apparatus. Thus, measures are taken in current ink jet printing apparatuses to overcome this problem. These measures include providing a structure for removing possible static electricity from members to which the ink mist is likely to adhere or avoiding the possible adhesion of the mist. For example, a fan may be installed for preventing the possible residue of the ink mist, or installation of a device that electrostatically sucks ink droplets.
However, taking these measures to collect or remove the ink mist adds extra costs to the product. Thus, fundamental solutions have been required which reduce possible satellites when ink droplets are ejected.